
Cosmic Theories  Kepler and NewtonKepler and his LawsJohannes Kepler (15711630) spent a great deal of time trying to understand the physics of the cosmos. After Copernicus and Tycho he was able to think in terms of the planets moving free of celestial spheres around the Sun. Using the motions of Mars and detailed observations from Tycho, he eventually devised his three laws of orbital motion:
Kepler realised that Copernicus's Suncentred cosmos must be real, but debate continued to rage until Newton eventually confirmed that Kepler's theory was consistent with his newlyformed theory of Gravity.
a is the semimajor axis, b is the semiminor axis of the ellipse. The Sun is at one focus, the other focus is empty. The degree to which a is greater than b is the 'eccentricity' of the orbit. You can see that the planet moves more slowly when it is farther away from the Sun so that the area swept by the Sunplanet line remains constant per unit of time. Newton's LawsIsaac Newton (1643  1727) developed the first fundamental 'laws' of mechanics. It is worth reminding ourselves of these laws:
Newton provided a mathematical proof showing that, if these laws are true, Kepler's laws are also true. At last it was possible to explain and predict the motions of the Sun, Moon and planets based on a theory that involved natural physical objects and resonable "laws" of motion. At this point in the story we may say that 'ancient cosmologies' gave way to modern science. For an interactive demonstration of the consequence of Newton's laws, go to the Orbit Simulation area of the website.


